When multiple operations are performed on sheet or flat plate objects of various dimensions, in particular in the production of printed circuits for electronic equipment, there is a need to perform operations on the objects themselves by machines of various kinds, hereinafter referred to in brief as functional machines. Such machines can have a conveyor member capable of transferring the objects in question, hereinafter generically referred to as sheet units or sheets through the working components of the machine in question. There is a need to load these sheet units onto such a conveyor from a cumulative collection of the sheets, normally in the form of a stack.
This loading may be performed manually, but in order to obtain greater productivity it is desirable to use an automatic device which performs the operation independently.
For this purpose it is frequently necessary to place the sheets in precise positions on the machine's conveyor, for example so that they engage with centering or securing means on the conveyor, such as pins, stop surfaces and the like, so as to convey the sheets through the machine in a geometrically precise position required by the intended operation.
In order to pick up the top sheet from a stack of sheets it is convenient to use pick-up members which sieze the upper surface of the sheet, for example by means of suction cups. In most cases this ensures that only one sheet is picked up, without interference from those beneath the lifted sheet.
However in some cases, particularly if the sheets have very smooth surfaces which fit together, it is possible that atmospheric pressure acting on the underside of the sheet below the top sheet which is to be picked up causes this sheet to remain attached to the top sheet, with an effect similar to that of a suction cup, thus causing it to be picked up together therewith.
A similar effect may also be due to the presence of lubricant or moisture between stacked sheets, or adhesion phenomena between the materials forming the sheets, or the like, depending upon the type of sheet in question.
This disadvantage, which can easily be overcome in manual operations, becomes a source of severe problems in the case of an automatic feed, because if it is not detected it results in the surplus sheet or sheets being dropped in an incorrect position before being deposited on the conveyor, or delivery to the machine of two or more stacked sheets, generally causing the machine to jam.